An American double agent has infiltrated the heart of the Real IRA - the terrorist organisation responsible for the Omagh bombing and blamed for recent attacks in Britain - in a spectacular intelligence coup by the FBI, MI5 and the Irish police.

The agent, David Rupert, has provided crucial evidence about the organisation and its suspected leader, Michael McKevitt, who was arrested by Irish police last month.

McKevitt is the first person to be charged with directing terrorism under a law introduced in Ireland after the 1998 Omagh bombing, which killed 29 people. Rupert is expected to be a key witness at the judge-only trial due to open in Dublin's special criminal court on April 24.

Rupert, a 49-year-old former lorry driver from Illinois, is understood to have first contacted the Irish Freedom Committee in Chicago four years ago.

With no permanent home, he described himself as a descendant of German immigrants and Mohawk Indians. He acted as a fundraiser and soon became the main representative in the US of the 32 County Sovereignty Committee, whose paramilitary is the Real IRA.

Rupert soon became disillusioned with the Real IRA and started providing information about his activities to the FBI and MI5.

But his work in the US had so impressed McKevitt and his wife, Bernadette - the sister of Bobby Sands, one of the IRA hunger strikers who died in 1981 - that he was invited to Ireland.

While from 1998 the FBI and MI5 used information provided by Rupert to monitor the movement of Real IRA funds to a bank in the Irish border town of Dundalk, McKevitt's home town, Rupert was addressing Real IRA rallies in the town and across Ireland.

Security sources say McKevitt took to Rupert and introduced him to real IRA leaders, including the army council. Rupert passed information about the group's leaders and meetings to the FBI and MI5 which began to share the intelligence with the Garda, the Irish police.

The Garda are believed to have bugged Real IRA meet ings and secretly filmed its leaders.

Then last year, McKevitt allegedly asked Rupert to commit a terrorist offence. It was the prospect of this evidence being produced in court that prompted the Irish police to ask the FBI and MI5 if they could "blow" their agent.

After discussing the matter with him, Rupert agreed to be a witness at the trial. The FBI and MI5 also agreed, in effect ending Rupert's role as a double agent.

Security sources are confident that the joint FBI-MI5 operation will have a serious long-term impact on the Real IRA. Liam Campbell, regarded as its main strategist, is also facing terrorist charges.

The Real IRA, which is accused of planting the recent taxi bomb outside the BBC in west London as well as last year's attack on MI6 headquarters, still remains a threat, security sources say.

It is believed to have a small number of activists in Britain equipped with explosives and weapons. It is also believed to have bought arms from the Balkans.